Les stocks de clupéidés situés le long des côtes ouest-africaines vivent et se reproduisent dans des zones d'upwelling saisonnier ou permanent. Ce sont des milieux fluctuants où la productivité est très variable d'une zone et d'une saison à l'autre. Les principales nurseries sont distribuées de façon discontinue le long des côtes ouest-africaines et ne sont pas toujours situées dans les zones les plus productives. Les reproductions les plus intenses sont localisées là où le plateau continental s'élargit et où la côte s'incurve par rapport à la direction prépondérante des vents. Ces particularités topographiques, communes aux zones de reproduction, minimisent localement la dérive vers le large de la couche d'eau superficielle. En Côte-d'Ivoire et au Congo, l'activité reproductrice est maximale durant la saison d'upwelling. Le long de la façade atlantique, la reproduction est parfois décalée par rapport à l'upwelling, elle est maximale avant et/ou après la saison d'upwelling. Les périodes d'abondance duzooplancton n'étant pas toujours en phase avec l'upwelling, les saisons de reproduction semblent être décalées par rapport à l'upwelling afin d'offrir des conditions optimales pour le développement des larves. Un schéma théorique est proposé pour illustrer les stratégies spatio-temporelles de la reproduction suivant l'intensité de l'upwelling, en considérant deux facteurs limitants: la production planctonique et la stabilité du milieu. L'intensité des upwellings est variable d'une année à l'autre. Au Sénégalles stratégies de reproduction répondent à cette variabilité en modifiant le calendrier de la reproduction suivant l'intensité de l'upwelling. De nombreux faits ne valident pas le schéma théorique proposé. Ces écarts au modèle peuvent être perçus comme des facteurs de régulation potentiels face au caractère non prévisible de l'environnement. Dans une perspective évolutioniste, ces exemples illustrent l'adaptabilité des stratégies de reproduction.

Data on the diet of species are important for understanding ecosystem dynamics and are fundamental for the implementation of recent approaches in stock assessment and consequently for the establishment of more ecological management measures. In mainland Portugal, as in most European countries, skates and rays represent an important proportion of commercial landings. The four main species landed are Raja clavata and Raja brachyura, followed by Leucoraja naevus and Raja montagui. This paper analyses their diets based on the examination of stomach contents. Food items were identified to the lowest identifiable taxon and were further assembled into major taxonomic groups designated as prey. Intra- and interspecific comparisons were made according to size and sex. All four species had generalized diets with differences in prey preference among them. Decapods and bony fish were the most frequent prey. Furthermore, an ontogenetic dietary shift was evident in all species at around 45−55 cm total length. Both intra- and interspecific differences observed seem to be related to size and morphological characteristics of the species, as well as type of dentition. These variations allow different species, as well as small and large specimens from the same species, to exploit a larger diversity of habitats.

Among the potential biotic factors affecting population dynamics, parasitism has received relatively little attention. The purpose of this study was to inventory marine bivalve parasites and to investigate relations between different parasite communities. Five intertidal stations along the French Atlantic shore were studied between Brittany and Arcachon Bay. Two bivalves (the edible cockle, Cerastoderma edule, and the Manila clam, Ruditapes philippinarum) were sampled and examined. The parasites included a bacterium (Vibrio tapetis), a protozoan (Perkinsus sp.), and digenetic trematode metacercariae (10 species). Perkinsus sp. prevalence and density in clams were higher than those measured in cockles (30 250 vs. 36 cells g−1 of gill wet weight, respectively) while digeneans were more common in cockles (46 metacercariae per cockle host vs. 1 per clam host). Distributions of digeneans and Perkinsus sp. were similar along the Atlantic shore. However, at each sampling station and at the individual scale, no correlation between Perkinsus sp. and digenean abundance was detected. These results suggest different ecological niches used by these two parasites within a community of bivalves but a similar response to environmental factors such as temperature and salinity. Vibrio tapetis was found at relatively high prevalence in all stations for both bivalve species (from 17% to 43% in cockles and from 23% to 50% in clams), but was associated with a low prevalence of Brown Ring Disease (BRD), a shell disease caused by this pathogen. No statistically significant relationship of these parasitic bacteria with trematodes and Perkinsus sp. was evidenced in the present study.

Domoic acid is a neurotoxin responsible for the Amnesic Shellfish Poisoning (ASP). With the aim of determining its depuration kinetics in raft mussels Mytilus galloprovincialis and the effects of body weight, salinity and temperature on it, an experiment involving these factors was carried out. Mussels which had incorporated this toxin to a concentration of 153 μg·g–1 of soft tissue during a bloom of the pennate diatom Pseudo nitzschia australis, were collected from the Galician Rías and placed at temperatures of 18 and 22 ºC and salinities of 12.5 and 31. Mussel samples were taken at the start of the experiment and daily during the four subsequent days and the domoic acid contents of the soft tissues were analysed. The simple one-compartment kinetics, that is a kinetics in which all toxin depurates at the same rate, was fitted to the data, with good quantitative results. Notwithstanding, the deviations of the model from the actual data were dependent on time, suggesting that such a simple model is not enough to correctly describe the data, and that a more complex kinetics may be more adequate. A two-compartment kinetic model, in which two pools of toxins exist (compartments), each one depurating at different rate, described qualitatively better the depuration but its quantitative contribution to the fit was not statistically significant. The parameters of the model, obtained by least squares fitting, suggest the possibility of a small second compartment of very small or null depuration rate, as detected in other species. This kind of model would explain the reduced quantitative contribution of the second (slowly depurating) compartment.Low salinity was shown to reduce the depuration rate. The two other factors checked, temperature and body weight, nor any interaction had significant effect on depuration rate.

Brown trout (Salmo trutta) were selected for growth for 4 generations. We tested the effects of selection on voluntary feed intake measured by self-feeders, feed efficiency and size variability. The specific effects of a slight feed restriction and of food deprivation were also investigated. Fish were issued from groups of eggs of selected females fertilised with sperm of selected (S group) or control males (S1/2 group). According to the growth rates expected for the selected and control lines, the S1/2 group was fertilised 12 days before the S group, so that all the fish reached 8 g at the same time. At 8 g, they were allotted to 8 tanks (500 fish per group) and 3 experimental periods followed. Fish were accustomed to self-feeders during a 28 days pre-experimental period. Then, half of the groups were fed ad libitum, and half were restricted (80% of the expected ad libitum level) for 171 days; growth and feed intake were recorded regularly and any uneaten food was weighed. Then followed a 56 days starvation period. At the end of the pre-experimental, feeding and starvation periods, individual weights and lengths were measured on 50 trout per tank. The response to selection at the end of the feeding period varied with the feeding level. In ad libitum fed groups, the mean final body weight of S was +6.1% higher than that of S1/2 and feed efficiency was similar (1.10 for S and S1/2). The higher growth of S compared to S1/2 was related to a higher feed intake of the S groups (+5.3%). When fish were restricted, the final body weight was lower in S (117.1 ± 2.1 g) than in S1/2 groups (123.8 ± 1.7 g). This was mainly related to a slightly lower feed efficiency of S compared to S1/2 at the beginning of the feeding period. Neither the group nor the feeding level affected the size variability of the fish. At the end of the starvation period, the relative loss of weight was equivalent for all the groups, and the variability of the weight was higher for S than for S1/2. The results highlight the fact that genetic gain can only be expressed when brown trout are fed ad libitum.

This study examines the trophic status and relationships of coral reef fish in the lagoon of New Caledonia, southwest pacific. The feeding habits of 34 fish species collected at three contrasted sites were first described using a compilation of gut contents observations and data from the literature. The carbon and nitrogen isotope signatures of these fish and of some of their potential ultimate food sources were also determined at each site. Despite some spatial variations in the isotopic signatures of most food sources and fish trophic groups, the overall trophic structures of fish assemblages were similar at the three sites. Stable isotope data were then used to re-assign fish species to trophic groups based on the δ 15N signatures of fish and their food sources. Herbivorous fish species were clearly distinguished from the other trophic groups by their lower δ 15N signatures, consistent with an estimated trophic position of ~2 for all species examined. Scaridae were however characterized by relatively higher δ 13C and lower δ 15N, which is probably linked with the role of detritus in their diet. The estimated trophic positions of planktivorous fish species were consistent with their gut contents-based classification. Conversely, the isotopic signatures of carnivorous and piscivorous fish species largely overlapped, and their estimated trophic positions were much lower than expected. This suggests that these species feed over a broader range of trophic levels and food sources than implied by the gut contents observations, and indicates that their diet is partly omnivorous. Finally, the relationships between body mass and the isotopic signatures of four fish species were significant for at least one isotopic ratio for each species. Since ontogenetic variations and omnivorous diets are difficult to assess with gut contents data only, stable isotopes revealed essential in estimating the actual trophic status and relationships characterizing the fish species under study.

Field studies describe echinoplutei not only as grazers but also as prey of naturally occurring fish and shellfish larvae. This finding suggests their potential as live feed in aquaculture. This paper reports on consistent spawnings of the captive sea urchin Paracentrotus lividus (Lamarck 1816) (Echinodermata: Echinoidea) throughout the year using diets of plant origin (yellow maize and/or dried seaweed) with fixed environmental conditions equivalent to field conditions during late spring (14 h of daily illumination and 18 °C of temperature). Broodstock maturation without unwanted spontaneous spawnings was achieved in two ways: extending the natural season of reproduction and inducing out-of-season wild specimens to mature. Controlled spawnings of captive sea urchins were induced every month of the year by KCl 0.5 M injections. The diet maize/seaweed combination gave the best results (79% of the tested urchins) in terms of consistent large spawnings throughout the year, followed by the pure maize diet (50%) and the pure seaweed diet (36%). When out-of-season wild sea urchins were induced to maturation, the majority (72%) of tested individuals required at least 60 days to spawn under KCl injection when fed the combination diet. The results demonstrate the feasibility of producing larval P. lividus in that high numbers of fertilized eggs (up to 5 million per female) can be obtained year round. The main limitation of exploiting P. lividus as planktonic feed seems to be the mortality of broodstock after injection with 1 ml KCl 0.5 M, which prevents reutilization. The 1-month post-injection survival rate was 30 ± 8% (mean ± SE). All surviving sea urchins spawned again after re-injection 1 month later, with a 1-month survival rate of 29%.

Duplicate groups of first-feeding (4-5-d-old) Tilapia zillii (Gervais) were maintained on two feeding regimes (high and low ration size) for a total period of 550 d (~ 17 months) in environmentally-controlled aquaria. Differences in mean individual fish weight were evident between fish fed the different ration sizes after just 3 months, though these differences were not statistically significant until day 180. Thereafter, fish maintained upon the higher ration were significantly larger than fish fed a reduced ration. Mortality in high ration fish over the course of the experiment was ~60 % compared to ~88 % in low ration fish. Mortality in both treatment groups was most pronounced during the first three months (particularly during the first 30 d) but was much reduced thereafter. Spawning trials undertaken from day 450 onwards indicated that there was no significant difference between the two ration groups in terms of spawning periodicity over a discrete time period of 60 d. Mean total fecundity was significantly higher in fish on high rations though mean egg diameter and gonadosomatic index remained unchanged. Adjustment of spawning data to a common maternal fish size using one-factor ANCOVA failed to detect significant differences between the two ration levels in terms of either mean total fecundity or mean egg diameter. Stereological analysis of plastic-embedded ovarian tissue revealed that the ovaries of low ration fish possessed significantly more stage 2 and 3 (early and late perinucleolar) oocytes and significantly less stage 6/7 (late-vitellogenic/maturing) oocytes than the ovaries of fish fed the higher ration. No significant differences were detected between the two rations in terms of ovarian atresia. It is suggested that under prolonged food restriction, female T. zillii sacrifice somatic growth such that reproductive investment can be maintained.

Dramatic changes in the Lake Victoria (East Africa) environment were observed after the introduction of the Nile perch (Lates niloticus) in the 1950s. An extraordinary spectrum of endemic haplochromine fishes, a result of intralacustrine adaptive radiation was reduced by massive species extinctions (ca. 65%) due in part to predation by Nile Perch. Such an abrupt destruction of natural diversity has never before been documented by scientists. Lake Victoria's haplochromine species flock comprised upwards of 400 species (5% of the world's known freshwater fishes), encompassing a remarkably wide trophic spectrum and constituting 83% of the lake's total fish biomass. The lot evolved in an isolated part of the Nile system since the formation of the lake basin about 750 × 103 years ago, but quite possibly as recently as 14 × 103 years ago, when most of the lake dried up. More than 50% of the haplochromine species (by number) were phytoplankton-zooplankton-detritus consumers; 55% of their biomass were detritivores and 27% zooplanktivores. The piscivore Nile perch (Lates niloticus) was first introduced into Lake Victoria in 1954. It underwent rapid population expansion in the 1980s, accompanied by haplochromine decline. Consequently, phytoplankton and detritus consumption by fishes was reduced. The biomass of the endemic Cyprinidae Rastrineobola argentea increased, as did its fishery, and predation pressure on zooplankton was therefore intensified. The population of the prawn Caridina niloticus became very dense, mostly in deep waters. The fishery and fish industry were altered fundamentally. Limnological changes suggesting eutrophication have been observed since 1960: hypolimnetic anoxia increased and the period of extensive vertical mixing was restricted to about one month per year; phytoplankton productivity increased and shifts from diatom to blue-green dominance occurred. Increased inputs of N (from the 1920s) and P (from the 1950s), induced through precipitation and human activity in the catchment area (agricultural and urban developments, deforestation, etc.) and high water levels accompanied by decline of available silicon, have persisted. Both top-down (Nile perch piscivory) and bottom-up (nutrient changes) influences enhanced eutrophication. The concurrent system changes in nutrient dynamics may have contributed an additional impact to the extinctions of haplochromine fishes.

A 200-kHz split-beam echosounder (BioSonics, DT6000) with a 6° circular-beam transducer was applied in a mangrove channel in Northern Brazil to study the migratory patterns of intertidal fish. Acoustic sampling was conducted horizontally across the channel perpendicular to the tidal current during two lunar cycles in the dry season 2000 and the wet season 2001. The complex acoustic environment of the mangrove channel was characterized by small target sizes (juvenile fish), multiple targets (aggregated fish), high reverberation and background noise levels due to sediment loads, plankton and mangrove litter transport. Dry seasons provided less noisy acoustic conditions resulting in clearer echo data than wet seasons. Neap tide data were less complex than spring tide data. During a tidal cycle, low water provided the clearest acoustic conditions. Mangrove leaves generated fish-like echoes. Analysis of two dry season wax moon cycles revealed fish flux maxima at low water, flood start and high water in the daytime and the night cycle. Night fish fluxes were significantly higher than at daylight. Throughout the tidal cycles, 60% of the fish traveled with the tide and 40% against, suggesting active foraging against the tide to be a major component of fish movements. Resident mangrove fish entered the intertidal creeks at early flood tide, leaving at late ebb tide at fairly shallow-water depths. Estuarine fish required a minimum water depth (about 2 m) for tidal migration. Since time delays during spring tides between immigration of resident and estuarine fish were reduced, foraging time and habitat accessibility would be enhanced and fish catches and fishes’ feeding success would be greater.

The impetus for this study was the mass bleaching event of giant clams in 1997–1998 at several reefs in the Great Barrier Reef (GBR, Australia). From September until December 1999, the study investigated the effects of high light intensities and increased temperature accompanied by a nutrient limitation on the metabolism of Tridacna gigas, to test, if these environmental changes can induce bleaching. In a 50-day trial similar conditions of the mass bleaching event were imitated stressing clams, collected from Orpheus Island and Nelly Bay, by a two- to threefold light intensity and an increased temperature of 4–6 °C. The objectives of the experiments were to determine whether high light intensities can induce changes in chlorophyll content or alter zooxanthella cell sizes and populations in the tissue. After 50 days of exposure to high light intensity the mantle tissue of stressed clams exhibited a decreased number of zooxanthellae per unit area from 19.8 ± 0.8 (× 107·cm–2) to 0.2 ± 0.2 (× 107·cm–2) (mean ± CL). Additionally, the average cell size of zooxanthellae were downsized from 7.4 ± 0.1 μm to 5.3 ± 0.1 μm (mean ± CL). Subsequently, the chlorophyll content of both, chl a and chl c1, declined as well, chl a from 192 ± 4 to 0.1 ± 0.1 μg·ml–1 and chl c1 from 145 ± 6 to 0 μg·ml–1 (mean ± CL). This study shows that increased light intensity and temperature are the main causes for bleaching in giant clams. Thus, the study confirmed the four major aspects involved in bleaching: (1) loss of symbiotic algae, (2) decrease of chl a/c1 in the remaining symbiotic algae, (3) retention of small zooxanthellae in the tissue and (4) release of ammonium (NH4+) into the water column while nutrient uptake of ammonium was largely blocked.

In this study, we compared the size selectivity of a T90 codend (nettingturned by 90 degrees) with that of a standard codend made of similar netting. Sea trialswere conducted in a Norway lobster directed fishery in the Kattegat-Skagerrak area, wherethere is a need for improved selectivity because of a severe discard problem. The codendswere tested by fishing simultaneously with them in a twin trawl rig. Codend covers mountedwith Minisamplers were used, which made it possible to catch individuals escaping duringtowing and haul-back separately. Herein we proposed a model to assess the sequentialselection during towing and haul back. This model takes into account the parameterCtow, which can be interpreted as theproportion of fish that comes into contact with the codend meshes during towing and,thereby, has a chance of escape. Compared to the standard codend, the T90codend retained fewer Norway lobster both below and above the legal minimum landing size(40 mm, cephalothorax length), thereby causing a reduction of commercial catch. Thedifference was mainly due to a significantly higher escape rate during towing for theT90 codend. For plaice below minimum landing size (27 cm), theretention was slightly but significantly higher for the T90 codendcompared to the standard codend. A model developed for both codends showed that not allplaice are able to attempt escapement during the towing process. For cod, the resultsindicated an increased L50 (the length at which 50% of this species iscaught) for the T90 codend, but the effect was not statisticallysignificant, probably due to the limited number of cod retained during the sea trials. Theresults demonstrated that, for all three species, a significant proportion did escapeduring haul-back in both codends.

Ce travail analyse l'aire de répartition marine actuelle de l'esturgeon européen Acipenser sturio à partir de 179 déclarations de captures accidentelles (dont 125 recaptures) correspondant à la période 1980–1994. Cette espèce menacée se rencontre du sud du golfe de Gascogne jusqu'à la Scandinavie et dans l'ensemble des îles Britanniques. Il est démontré que cette aire de répartition est bien celle de la population se reproduisant dans le bassin versant Gironde-Garonne-Dordogne. Les captures sont faites sur des fonds de moins de 100 m, sur des substrats variés et de façon équivalente par des chaluts de fond, des filets maillants et des filets tramails à l'occasion de la pêche de poissons ou de crustacés benthiques. Les esturgeons capturés sont surtout des immatures mesurant en moyenne 113 cm (min. 35 cm, max. 244 cm). Les captures ont principalement lieu durant les 6 premiers mois de l'année et sont en phase avec celles observées dans l'estuaire interne de la Gironde. Ce type de données, malgré des limites liées à leur naturemême, permet dans le cas d'espèces rares et/ou migratrices d'obtenir des informations intéressantes et inaccessibles par ailleurs sur leur écologie marine.

The toxic dinoflagellate Alexandrium catenella recurrently blooms on thecoasts of France and produces Paralytic Shellfish Toxins (PSTs) that accumulate inbivalves. These toxins can affect various physiological functions including reproduction.The present study aims to validate measurements of sperm viability, DNA content andmitochondrial membrane potential in Pacific oyster Crassostrea gigasusing flow cytometry coupled with fluorescent markers, and to use thesemeasurements to assess the cellular parameters of sperm from Pacific oysters exposed toA. catenella. These parameters may influence fertilization,embryogenesis and larval development in free-spawning shellfish. Sperm viability and DNAcontent estimation were assessed using SYBR-14, which only penetrates cells with intactmembranes. Cell mortality was measured with propidium iodide (PI), which penetrates cellswith membrane damage. Mitochondrial membrane potential, used as an estimate ofmitochondrial function, was measured using JC-1 dye, which selectively enters intomitochondria and reversibly changes colour from green to orange as the membrane potentialincreases. To assess the effect of toxic algae on oyster sperm, broodstock (ripe oysters)were fed toxic (A. catenella) or non toxic (Heterocapsatriquetra) dinoflagellates at 250 cell ml-1 for 9 days. After thisexposure period, mature oysters were stripped and cellular responses of sperm analysed.Average DNA staining, as measured by SYBR-14, appeared lower and more variable in gametesfrom A. catenella-exposed oysters than in those from control oysters fedH. triquetra. Additionally, mitochondrial membrane potential of spermfrom A. catenella-exposed oysters was significantly higher (1.5 fold)than that of sperm produced by oysters fed H. triquetra. Both theincrease of mitochondrial membrane potential and the modification of DNA structure can beexpected to impact spermatozoa ability to fertilize oocytes and could thus impact relatedreproductive processes.

Adult oysters Crassostrea gigas were experimentally fed with Alexandrium catenella and Alexandrium minutum which are responsible for recurrent toxic blooms in French coastal waters. C. gigas produced faeces and pseudofaeces containing intact and viable temporary pellicular cysts of these two Paralytic toxin producing species. When incubated in favourable conditions, these pellicular cysts were able to germinate at high rates (between 74 and 94%) and the resulting vegetative cells divided with growth rates close to the non- ingested cells (control). The toxin profile of the vegetative cells originated from the germinated temporary cysts was analyzed by liquid chromatography/fluorescence detection. Total toxin content of newly germinated cells was lower than that of cultured cells. Besides, cell contents of C2, B1, B2 and dcGTX3 toxins featured some changes. Our results suggest that the increased spreading of toxic dinoflagellates through the transfer of shellfish from contaminated towards pristine coastal areas cannot be ruled out. We also suggest that pellicular cysts and newly germinated cells could represent a potential way for the transfer of paralytic toxins toward the higher trophic levels.

Several experiments using a self-regulated system were conducted to define the factors likely to influence the uptake of paralytic shellfish poison (PSP) by oysters in the Penzé estuary (France, Brittany). Each 4-day experiment was carried out in a recirculated sea water system using 15 Pacific oysters (Crassostrea gigas) separated from each other and supplied with unfiltered natural seawater containing alternatively toxic (Alexandrium minutum) or non-toxic (Skeletonema costatum) algal diets. The food supply and exposure times to toxic diets were determined according to field studies of the upstream and downstream movement of patches containing A. minutum. The experimental parameters corresponded roughly to the hydrological conditions generally observed in June when tidal coefficients are lowest and blooms occur: (i) A. minutum concentrations in sea water of 200, 5000 and 10 000 cell ml−1; (ii) inorganic matter consisting of 5 and 15 mg L−1 of calcinated muddy sediments; and (iii) low and high tide salinities of 25 and 35‰, respectively. Significant experimental contamination (greater than the 80 µg STX equiv. 100 g-1 sanitary threshold) occurred after 4 days of exposure for the monospecific A. minutum diet (20-200 cell ml−1) and alternated A. minutum and S. costatum diets (5000 and 20 000 cell ml−1, respectively). Contamination levels were less than the sanitary threshold for alternated A. minutum/S. costatum diets of 200 and 20 000 cell ml−1, respectively, and for a monospecific A. minutum diet (1000−10 000 cell ml−1). In the last case, the accumulation rate was quite low, possibly because of inhibition of the filtration rate related to a lower biodeposit production rate and decreased feeding time activity. The addition of inorganic matter appeared to play a significant role in the observed increase of toxin uptake, whereas salinity was not a determining factor for toxin accumulation rates. These last observations were corroborated by statistical analysis and stepwise multiple linear regressions integrating all or some of the experimental parameters.

Improved inland culturing methods for marine invertebrates are the most important prerequisite for the establishment of inbred lines. Here, we study the effects of improved diet on growth rates, survivorship and reproductive activities of the colonial urochordate Botryllus schlosseri, an important model species in a variety of scientific disciplines. Six long-term experiments (6−8 weeks each; 3−8 treatments per experiment) were conducted in an attempt to evaluate 10 different food types alone and in various combinations. We took a hierarchical approach in which some food types were contrasted and the results were used to design the next experimental set with new food types. A mixture of at least two types of diets was superior to any monotonous diet examined. Results were also characterized by high variability between colonies of the same hatch in any studied life history aspect, by the appearance of reproductive sterile colonies in the cultures, high survivorship, fast growing and long-lived colonies. The improved maintenance protocols enabled us to develop the first long-lived (> 6.5 years) inbred line of Botryllus with four successive generations of self-crossed pedigree animals.

Juvenile oyster disease (JOD) in the eastern oyster, Crassostrea virginica, is characterized by a conchiolin deposit on the inner surface of the valves. Similarities to pathological syndromes affecting other oysters (pearl oyster mortality) and clams (brown ring disease), has suggested an infectious origin and a possible bacterial etiology. Bacteriological analysis (Total Heterotrophic Bacteria, THB and Total Vibrio sp., TVS) of oyster soft tissues, shell fluid and inner shell surfaces were monitored during the course of JOD onset and development. A significant increase in THB (but not TVS) occurred in shell fluid and at thesurface of the inner shell of JOD affected oysters. At the same time, changes in cytometric parameters (Total Hemocyte Counts, THC; Differential Hemocyte Counts, DHC) in hemolymph and shell fluid were documented in symptomatic oysters. THC in the shell fluid showed a decrease in late-stage JOD oysters. The disease was also correlated with altered cell ratios in both hemolymph and shell fluid, resulting in an increase in the percentage of the granulocytes in both locations. Our results have showed that main pathological changes associated with JOD occurred in extrapallial region rather than in the soft tissues. The disease was transmitted in the laboratory by extracts from diseased oysters: anomalous conchiolin developed within 4 weeks and the prevalence was higher when two inoculations were performed rather than one. A number of bacterial strains were found at high levels in diseased oysters and isolated. However, none produced the JOD deposit when injected into asymptomatic oysters. The bacterial etiology hypothesis for JOD should be explored further, expanded beyond the family Vibrionaceae, and should include consideration of a multiple etiology.

Introgression between introduced domesticated stocks and natural populations of brown trout was investigated by protein electrophoresis in three Mediterranean rivers. From each river, one undisturbed site and one, closely located, yearly stocked site were sampled. Comparison of the electrophoretic variation observed in samples from the undisturbed sites with that in the introduced hatchery strains revealed several specifically "domestic" variants. The genetic control of these variants was demonstrated by breeding experiments. The samples collected from the stocked sites showed introgression rates ranging from approximately 0 to 40%. The genotype frequencies observed in these samples suggested that no reproductive barrier exists between introduced and native stocks and that they form random mating populations.

The present study investigates vertical distribution of fish size in three 12 m  ×  12 mwide and 14 m deep sea cages stocked with Atlantic salmon of average weight from 3.5 to3.7 kg, at commercial densities between 15.6 to 16.2 kg m-3, in Norway. Foreach cage, individual fish weight were estimated by three 0.6 m  ×  0.6 m measuring framesas fish swam through. The frames were positioned at 3, 6 and 9 m depth. Recordings werecarried out over five days per cage in succession, and during relatively stableenvironmental conditions in late autumn 2010. In all cages, measured fish were 15−25%smaller at 3 m compared to the average weights at 6 and 9 m depth. Largest average weightdifference between depths within one cage was 0.995 kg. The average weight at 6 and 9 mwas higher at night-time compared to daytime. Fish at the lower end of the weight spectrumwere predominantly registered at 3 m, while fish at the higher end of the spectrum weremostly registered at the greater depths. Fish of average size were well represented at allthree depths. The fact that smaller fish swam shallower may be ascribed to naturalbehavioural traits and introduce an important consideration in representative samplingwithin commercial sea cages. In biomass estimations and sea lice counts size-dependentvertical stratification needs to be accounted for.